1. Field of the Invention
The invention relates to a steering wheel rotational angle detecting apparatus for detecting rotation of a steering wheel based on rotation of an annular rotating member that rotates in accordance with rotation of the steering wheel.
2. Description of the Related Art
A conventional apparatus for detecting rotation of a steering wheel is shown in FIGS. 10 and 11. An annular coupling member 3 that also serves as a member for detecting rotational angle of a steering wheel 2 is disposed in a central portion of a lower face of the steering wheel 2, which is fixed to an upper end of a steering shaft 1. The steering shaft 1 extends through the coupling member 3.
Three pins 3a are formed on an upper end portion of the coupling member 3, arranged along the circumference of the end portion as shown in FIG. 11. The pins 3a are fitted into recesses 2a that are formed in the positions on the steering wheel 2 corresponding to the pins 3a. By means of the pins 3a thus fitted into recesses 2a, the coupling member 3 rotates as the steering wheel 2 is turned.
A rotational angle detector 4 is disposed on a lower end portion of the coupling member 3 and surrounds the steering shaft 1. The rotational angle detector 4 has an annular rotating member 6 that is rotatably disposed in an annular case 5. The annular case 5 is fixed to, for example, a steering column (not shown). The annular rotating member 6 is provided with an engaging pin 6a protruding radially inward. The engaging pin 6a is disposed to engage with an engaging cut-out portion 3b that extends in a lower end portion of the coupling member 3 in the direction of its longitudinal axis.
A ring-like slitted plate 7 is fixed to an outer peripheral portion of the annular rotating member 6. The slitted plate 7 has a plurality of slits that extend in the circumferential direction. A photointerrupter 8 is disposed on the case 5 and covers an outer peripheral portion of the slitted plate 7.
When the steering wheel 2 is turned, the coupling member 3 correspondingly turns so that the annular rotating member 6 of the rotational angle detector 4 also rotates. As the slitted plate 7 of the annular rotating member 6 rotates, the photointerrupter 8 detects light reception signals generated by light passing through the slits of the slitted plate 7. Based on the level and phase of the light signals, the rotational angle detector 4 obtains a signal corresponding to the rotational angle and direction of the steering wheel 2, thus detecting rotation of the steering wheel 2.
Since it is difficult to completely align the rotational axis of the rotational angle detector 4 with the rotational axis of the steering shaft 1 when mounting the rotational angle detector 4, some error in the mount precision is normally inevitable. Therefore, to ensure that the annular rotating member 6 of the rotational angle detector 4 will smoothly rotate even if the rotational axes of the two components are not completely aligned, dimensional allowances in the rotational directions are normally provided in, for example, gaps between the engaging pins 3a of the coupling member 3 and the engaging recesses 2a of the steering wheel 2, and a gap between the cut-out portion 3b of the coupling member 3 and the engaging pin 6a of the annular rotating member 6.
However, such dimensional allowances cause problems. That is, the annular rotating member 6 of the rotational angle detector 4 may rotate following the steering wheel 2 as long as the steering wheel 2 is turned in one direction, whereas when reversed, the steering wheel 2 rotates idly an amount corresponding to the dimensional allowances relative to the annular rotating member 6, and then re-engages with and turns the annular rotating part 6. This failure in immediate or precise transmission of rotation from the steering wheel 2 to the annular rotating member 6 of the rotational angle detector 4 hinders improvement of precision in detection of the rotational angle.